In recent years, as a result of advances in the development of high output fiber lasers, fiber lasers have begun to be used in a variety of fields such as processing machinery, medical equipment, measuring instruments and the like. In particular, in the field of material processing, because fiber lasers have superior light focusing abilities compared to other lasers and allow an extremely small beam spot having a high power density to be obtained, they make it possible for precision processing to be performed. Moreover, they also enable non-contact processing and processing of hard substances which are able to absorb laser light to be performed so that, as a result, the use of these fiber lasers is spreading rapidly.
Pulse output fiber lasers generally have an MO-PA structure in which comparatively low-power pulse light is emitted by a master oscillator (MO: Master Oscillator), and this pulse light is then amplified to a desired output by an optical fiber amplifier (PA: Power Amplifier). A schematic structural view of an MO-PA type of high output fiber laser is shown in FIG. 1. When it is not possible for pulse light to be amplified to a desired output using a single power amplifier, then in some cases a plurality of cascade-connected power amplifiers are used.
However, an MO-PA type of optical fiber laser, and particularly a high output fiber laser which outputs 10 W or more has the drawback that it is easily damaged by reflection light. For example, when processing is being performed using a fiber laser, there are cases when laser light output from the fiber laser is reflected by a surface of an object being processed, and a portion of this laser light gets returned to the fiber laser. Although this reflection light is faint, it is amplified as it passes through the PA towards the MO so that the power thereof increases, and there are cases when the optical components making up the MO and optical components located between the MO and the PA are damaged.
Moreover, pulse light becomes amplified in the PA, and during the time until the next pulse light is irradiated into the PA, ASE (Amplified Spontaneous Emission) light is output from the rare earth-doped optical fiber which is used in the PA. When this light is reflected by the object being processed and becomes irradiated once again into the PA, there are cases in which parasitic oscillation is generated. If parasitic oscillation is generated, then pulse light having an extremely high peak value is emitted from the PA towards the MO, and there are cases when the optical components making up the MO and optical components located between the MO and the PA are damaged by this pulse light.
As is disclosed, for example, in Patent document 1, consideration has been given to the use of an isolator in order to protect the optical components of the MO and optical components located between the MO and the PA from reflection light. Here, in a power amplifier which is used for optical communication, it is proposed that an isolator be used in order to prevent ASE light which is emitted from a latter-stage power amplifier being irradiated onto a prior-stage power amplifier. In the same way, in a fiber laser, it is possible to prevent reflection light being irradiated onto optical components located in the MO and between the MO and the PA by providing an isolator immediately in front of the PA.
[Patent document 1] Japanese Patent No. 2619096
[Patent document 2] Japanese Unexamined Patent Application, First Publication No. 2002-296630
[Patent document 3] U.S. Pat. No. 5,864,644
[Non-patent document 1] “Fabrication and characterization of an all-solid 2D photonic bandgap fiber with a low-loss region (<20 dB/km) around 1550 nm,” G. Bouwmans, OPTICS EXPRESS 17, Vol. 13, No. 21, 2005, pp 8452-8459